There is no risk to human health because the radioactive material was further diluted when it entered a storage basin before flowing into the lake, NRC spokeswoman Viktoria Mitlyng told The Associated Press. She said there is "absolutely" no risk to human health.

The plant is 80 miles east-northeast of Chicago in Van Buren County's Covert Township. It's been under heightened Nuclear Regulatory Commission attention because of a series of breakdown over two years.

The southwestern Michigan plant was shut down Sunday after officials discovered a growing leak the day before in a water storage tank.

Safety investigators reported Monday that 79 gallons from the 250,000-gallon tank leaked into a basin holding thousands of gallons of non-radioactive water, Mitlyng said. She said that water from the tank is no longer reaching the lake.

The "very slightly radioactive water" contained small amounts of tritium and may have had trace amounts of cobalt and cesium, she said. It was further diluted to the degree that it would have been undetectable at the point where it entered Lake Michigan, she said.

Mitlyng said the NRC is sending a metallurgy expert to the plant to inspect the leaky tank once it is fully drained and pinpoint the source of the leak.

The same tank sprang a leak in 2012, and some of the water caused water seepage into the plant's control room. Afterward, the NRC set a cap of 38 gallons a day on how much water could leak each day, Mitlyng said.

The leak rate went from about 1 gallon on Friday to about 90 gallons Saturday, she said, leading to the shutdown.

Palisades has been shut down nine times since September 2011, including in February for a different water leakage problem.

The amount of radiation the NRC says was released is near the background level — what is found occurring in the environment on a daily basis — and shouldn't raise any public concern, said Ronald Gilgenbach, chairman of the nuclear engineering and radiological sciences department at the University of Michigan.

"The NRC has a serious standard to protect the public," he said.

The public can generally count on the NRC's risk assessments and its willingness to get tough with operators of nuclear plants that have recurring problems, said Alan Jackson, a radiation health physicist at Henry Ford Health System in Detroit.

What should be of concern to regulators and the public, Jackson said, is whether any nuclear power plant has enough of a culture of safety in place. That's especially important because of the intense pressures in the electrical power industry to keep costs low.

Saturday's radiation leak "most likely ... is not a public health concern," Jackson said. "What's more a concern is why do these things keep happening?"

"I would fixate on, 'OK, you have this problem. Why aren't you fixing it?'" he said.